Power amplifier

ABSTRACT

The present disclosure is to improve the power added efficiency of a power amplifier at high output power. The power amplifier includes: a first capacitor with a radio frequency signal input to one end thereof; a first transistor whose base is connected to the other end of the first capacitor to amplify the radio frequency signal; a bias circuit for supplying bias to the base of the first transistor; and a second capacitor with one end connected to the base of the first transistor and the other end connected to the emitter of the first transistor.

This application is a continuation of U.S. patent application Ser. No.15/585,418 filed on May 3, 2017, which is a continuation of U.S. patentapplication Ser. No. 14/926,441 filed on Oct. 29, 2015, entitled “POWERAMPLIFIER”, which claims priority to U.S. Provisional Application Ser.No. 62/078,625, filed on Nov. 12, 2014, entitled “POWER AMPLIFIER”, theentirety of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a power amplifier.

Background Art

A mobile communication device such as a cellular phone employs a poweramplifier to amplify power of a radio frequency (RF) signal to betransmitted to a base station.

For example, Patent Document 1 discloses a power amplifier includingmultiple unit cells. Each unit cell includes an amplification transistorand a bias circuit for supplying bias to the base of the transistor.

CITATION LIST Patent Document

[Patent Document 1] JP2011-130066 A

SUMMARY OF THE INVENTION

As disclosed in Patent Document 1, such a structure to supply bias tothe base of an amplification transistor is common. However, as theoutput level increases, current flowing from the bias circuit into thebase of the amplification transistor increases, resulting in a decreasein power added efficiency (PAE).

The present disclosure has been made in view of such circumstances, andit is an object thereof to improve the power added efficiency of a poweramplifier at high output power.

A power amplifier according to one aspect of the present disclosureincludes: a first capacitor with a radio frequency signal input to afirst end of the first capacitor; a first transistor whose base isconnected to a second end of the first capacitor to amplify the radiofrequency signal; a bias circuit for supplying bias to the base of thefirst transistor; and a second capacitor with a first end connected tothe base of the first transistor and a second end connected to theemitter of the first transistor.

According to the present disclosure, the power added efficiency of apower amplifier at high output power can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a configuration example of atransmitting unit including a power amplification module as oneembodiment of the present disclosure.

FIG. 2 is a diagram illustrating the configuration of a power amplifier160A as an example of the configuration of a power amplifier 160.

FIG. 3 is a diagram illustrating a simplified equivalent circuit of thepower amplifier 160A, where the base-emitter junction of a transistor200 is set as a simple diode D and an input signal is represented as asquare wave signal.

FIG. 4 is a graph illustrating the voltage V_(B)-current IBcharacteristic of the diode.

FIG. 5 is a graph illustrating the voltage V_(B)-capacitance Ccharacteristic of the diode.

FIG. 6 is a diagram illustrating an equivalent circuit in the case ofvoltage V_(B)<on-voltage V_(ON).

FIG. 7 is a diagram illustrating an equivalent circuit in the case ofvoltage V_(B)>on-voltage V_(ON).

FIG. 8 is a graph illustrating examples of waveforms of the voltageV_(B).

FIG. 9 is a diagram illustrating an example of the structure of a unitcell that can be employed in the power amplifier 160.

FIG. 10 is a diagram illustrating the configuration of a power amplifier160B with multiple unit cells 300 connected in parallel.

FIG. 11 is a graph illustrating simulation results when the capacitancevalue C_(CUT) of a capacitor 210 is 0.4 pF and the capacitance valueC_(ADD) of a capacitor 240 is 0.01 pF.

FIG. 12 is a graph illustrating simulation results when the capacitancevalue C_(CUT) of the capacitor 210 is 0.4 pF and the capacitance valueC_(ADD) of the capacitor 240 is 1 pF.

FIG. 13 is a graph illustrating simulation results that indicate arelationship between the capacitance value C_(ADD) of the capacitor 240and power added efficiency in the power amplifier 160B.

FIG. 14 is a graph illustrating simulation results when the capacitancevalue C_(CUT) of the capacitor 210 is 1.4 pF and the capacitance valueC_(ADD) of the capacitor 240 is 0.01 pF.

FIG. 15 is a graph illustrating simulation results when the capacitancevalue C_(CUT) of the capacitor 210 is 1.4 pF and the capacitance valueC_(ADD) of the capacitor 240 is 1 pF.

FIG. 16 is a diagram illustrating an example in which the capacitor 240is made up of diodes.

DETAILED DESCRIPTION

Embodiments of the present disclosure will be described below withreference to the accompanying drawings. FIG. 1 is a diagram illustratinga configuration example of a transmitting unit including a poweramplification module as one embodiment of the present disclosure. Atransmitting unit 100 is used, for example, in a mobile communicationdevice such as a cellular phone to transmit various signals such asvoice and data to a base station. Although the mobile communicationdevice also includes a receiving unit for receiving signals from thebase station, the description thereof will be omitted here.

As illustrated in FIG. 1, the transmitting unit 100 includes amodulation unit 110, a power amplification module 120, a front end unit130, and an antenna 140.

The modulation unit 110 modulates an input signal based on a GlobalSystem for Mobile Communications (GSM)® modulation system or the like togenerate an RF signal in order to perform radio transmission. Forexample, the RF signal ranges from about hundreds of MHz to several GHz.

The power amplification module 120 amplifies the power of the RF signal(P_(IN)) to a level necessary for transmission to a base station, andoutputs the amplified signal (P_(OUT)). For example, the poweramplification module 120 can be made up of two-stage power amplifiers.Specifically, as illustrated in FIG. 1, the power amplification module120 can include power amplifiers 150, 160 and matching circuits (MN:Matching Networks) 170, 180, and 190. The power amplifier 150 is afirst-stage (drive-stage) amplifier to amplify an input RF signal andoutput the amplified signal. The power amplifier 160 is a second-stage(power-stage) amplifier to amplify the input RF signal and output theamplified signal. The matching circuits 170, 180, and 190 are circuitsfor matching impedance between circuits, and each matching circuit ismade up using a capacitor and an inductor. Note that the number ofstages of power amplifiers that constitute the power amplificationmodule 120 is not limited to the two stages, and it may be one stage, orthree or more stages.

The front end unit 130 performs filtering on the amplified signal,switching to a received signal received from the base station, and thelike. The amplified signal output from the front end unit 130 istransmitted to the base station through the antenna 140.

FIG. 2 is a diagram illustrating the configuration of a power amplifier160A as an example of the configuration of the power amplifier 160. Thepower amplifier 160A includes an NPN transistor (hereinafter simplycalled “transistor”) 200, a capacitor 210, a bias circuit 220, aninductor 230, and a capacitor 240.

The transistor 200 (first transistor) is, for example, a heterojunctionbipolar transistor (HBT). Power-supply voltage V_(CC) is supplied to thecollector of the transistor 200 through the inductor 230, an RF signal(RF_(IN)) is input to the base of the transistor 200 through thecapacitor 210, and the emitter of the transistor 200 is grounded.Further, bias is supplied from the bias circuit 220 to the base of thetransistor 200. The transistor 200 amplifies the RF signal input to thebase and outputs the amplified signal (RF_(OUT)) from the collector.

The RF signal is input to one end of the capacitor 210 (firstcapacitor), and the other end of the capacitor 210 is connected to thebase of the transistor 200. The capacitor 210 cuts a DC component of theRF signal and outputs the RF signal to the base of the transistor 200.

The bias circuit 220 includes a transistor 250, resistors 260, 270, acapacitor 280, and diodes 290, 291. Battery voltage V_(BAT) is suppliedto the collector of the transistor 250 (second transistor), bias controlvoltage V_(CONT) is supplied to the base of the transistor 250 throughthe resistor 260, and the emitter of the transistor 250 is connected toone end of the resistor 270. The bias control voltage V_(CONT) isapplied to one end of the resistor 260, and the other end of theresistor 260 is connected to the base of the transistor 250. One end ofthe resistor 270 (first resistor) is connected to the emitter of thetransistor 250, and the other end of the resistor 270 is connected tothe base of the transistor 200. One end of the capacitor 280 isconnected to the base of the transistor 250, and the other end of thecapacitor 280 is grounded. The diodes 290, 291 are connected in series,where the anode of the diode 290 is connected to the base of thetransistor 250, and the cathode of the diode 291 is grounded. The biascircuit 220 outputs bias current I_(BIAS) to the base of the transistor200 based on the bias control voltage V_(CONT). The capacitor 280 canreduce noise input to the base of the transistor 250. The diodes 290,291 can reduce fluctuations in base voltage of the transistor 250 withrespect to variations in bias control voltage V_(CONT).

The power-supply voltage V_(CC) is applied to one end of the inductor230, and the other end of the inductor 230 is connected to the collectorof the transistor 200. The power-supply voltage V_(CC) is, for example,a predetermined level of voltage generated by a regulator.

The capacitor 240 (second capacitor) is, for example, MIM (MetalInsulator Metal) capacitance. One end of the capacitor 240 is connectedto the base of the transistor 200, and the other end of the capacitor240 is connected to the emitter of the transistor 200. For example, thecapacitance value C_(ADD) of the capacitor 240 is roughly equivalent orsubstantially similar to the capacitance value of the transistor 200 inan off state. The capacitor 240 is provided to improve the power addedefficiency of the power amplifier 160A at high output power.

First, operation when the capacitor 240 is not provided in the poweramplifier 160A will be described. As illustrated in FIG. 3, theconfiguration of the power amplifier 160A is simplified for illustrativepurposes. In FIG. 3, the base-emitter junction of the transistor 200 isrepresented as a simple diode D to focus attention on the base current.Further, for the sake of simplification, such a structure that the biasvoltage V_(BIAS) is applied through the resistor 270 and the RF signalis applied through the capacitor 210 is represented as a structure whereboth signals are applied through a resistor R. In addition, it isassumed that the input signal is a square wave signal. The resistancevalue of the resistor R has a high degree of freedom, which is, forexample, about 16Ω.

The characteristics of the diode are simplified as follows: Anequivalent circuit for the diode can be described as a parallelconnection of non-linear resistance and non-linear capacitance.

Although the voltage V_(B)-current I_(B) characteristic of thenon-linear resistance is principally expressed as an exponentialfunction as indicated by the broken line in FIG. 4, the non-linearresistance is represented here by a piecewise linear model in which thenon-linear resistance is completely turned off when the voltage V_(B) isless than on-voltage V_(ON) and takes a resistance value r_(d) when thevoltage V_(B) is higher than or equal to the on-voltage V_(ON) asindicated by the solid line in FIG. 4. When the power amplifier 160Aoperates with the maximum power, collector current flowing through thetransistor 200 is about 300 to 400 mA. Since the current amplificationfactor of the transistor 200 is about 100, base current is about 3 to 4mA. The resistance value r_(d) in this case is about 6Ω.

As indicated by the broken line in FIG. 5, when the voltage V_(B) isless than the on-voltage V_(ON), the capacitance value of the non-linearcapacitance is principally determined by bias voltage-dependent junctioncapacitance, while when the voltage V_(B) is higher than or equal to theon-voltage V_(ON), diffusion capacitance proportional to the exponent ofthe voltage V_(B) becomes dominant. The actual value of the non-linearcapacitance is represented with a curve as indicated by the broken linein FIG. 5. Here, it is assumed that the non-linear capacitance isconstant capacitance C_(j) when the voltage V_(B) is less than theon-voltage V_(ON) and higher constant capacitance C_(d) when the voltageV_(B) is higher than or equal to the on-voltage V_(ON). In the poweramplifier 160A, for example, C_(j)=0.6 pF and C_(d)>8 pF.

As illustrated in FIG. 6, when the voltage V_(B) is less than theon-voltage V_(ON), the diode D is represented as the capacitance C_(j).In this case, the time constant of the circuit is a value obtained bymultiplying a product of C_(j) and R by 2π, which is about 60 psec.

As illustrated in FIG. 7, when the voltage V_(B) is higher than or equalto the on-voltage V_(ON), the diode D is represented as a parallelconnection of the capacitance C_(d) and the resistance r_(d). In thiscase, the time constant of the circuit is a value obtained bymultiplying a product of C_(d) and r_(d)//R by 2π, which is about 220psec (in the case of C_(d)=8 pF).

Since the diode is turned on when DC bias is applied, it operates in anarea indicated by an equivalent circuit in FIG. 7. When the RF signal isapplied to this circuit, the circuit operates in the area illustrated inFIG. 7 in a range where the amplitude of the RF signal is small. Whenthe amplitude of the RF signal becomes large to some extent, the circuitoperates across two areas. In other words, the time constant becomessmall in the area where the voltage V_(B) is less than the on-voltageV_(ON), and the voltage V_(B) becomes a steep pulsed waveform asindicated by the broken line in FIG. 8.

Next, operation when the capacitor 240 is provided in the poweramplifier 160A will be described. This corresponds to a structure withcapacitance C_(ADD) added in FIG. 6 and FIG. 7. It is assumed that thecapacitance value of the capacitance C_(ADD) is 1 pF. In this case, thetime constant in the area where the voltage V_(B) is less than theon-voltage V_(ON) is 96 psec. The time constant in the area where thevoltage V_(B) is higher than or equal to the on-voltage V_(ON) is 247psec. The increase rate of the time constant is pronounced in the areawhere the voltage V_(B) is less than the on-voltage V_(ON). Therefore,the waveform of the voltage V_(B) when the RF signal is applied becomesas indicated by the solid line in FIG. 8, where the pulsed waveform isrounded with a prolonged period during which the voltage V_(B) is lessthan the on-voltage V_(ON). This means that the off-state periodincreases. This leads to a decrease in average current value and hencean improvement in efficiency.

Although the power amplifier 160A is illustrated in FIG. 2 as an exampleof the power amplifier 160, the power amplifier 160 can also be made upof multiple unit cells connected in parallel. FIG. 9 is a diagramillustrating an example of the structure of a unit cell that can be usedin the power amplifier 160. A unit cell 300 includes the transistor 200,the capacitors 210, 240, the transistor 250, and the resistor 270 in thepower amplifier 160A illustrated in FIG. 2. FIG. 10 is a diagramillustrating a configuration of a power amplifier 160B in which multiple(e.g., 16) unit cells 300 are connected in parallel. Even in the poweramplifier 160B where the multiple unit cells 300 are connected inparallel, since the capacitor 240 is provided in each unit cell 300,power added efficiency can be improved as mentioned above. Note that thestructure of the unit cell 300 illustrated in FIG. 9 is just an example,and elements included in the unit cell are not limited to theseelements.

Based on simulation results, the following describes that power addedefficiency is improved by the power amplifier 160B. FIG. 11 is a graphillustrating simulation results when the capacitance value C_(CUT) ofthe capacitor 210 is 0.4 pF and the capacitance value C_(ADD) of thecapacitor 240 is 0.01 pF. Note that C_(ADD)=0.01 pF is a value smallenough to ignore the capacitor 240. In other words, the simulationresults in FIG. 11 are equivalent to the simulation results when thecapacitor 240 is not provided.

In FIG. 11, the abscissa indicates time, and eight indexes are indicatedon the ordinate, where RF_(IN) denotes the voltage of the RF signalinput to the capacitor 210, I1 denotes current output from the capacitor210, I2 denotes current obtained by adding I_(BIAS) to I1, I_(B) denotesbase current of the transistor 200, I_(BIAS) denotes bias current outputfrom the bias circuit 220, I_(ADD) denotes current flowing through thecapacitor 240, V_(B) denotes the base voltage of the transistor 200, andV_(C) denotes the collector voltage of the transistor 200.

As indicated at point A1 in FIG. 11, when the transistor 200 is turnedoff at high output power (i.e., when the amplitude level of V_(C) ishigh), the base voltage V_(B) significantly drops. Along with this, thebias current I_(BIAS) increases as indicated at point B1. The higher thebias current I_(BIAS), the earlier the timing at which the transistor200 is turned on as indicated at point C1. From this, it is found thatpower added efficiency is reduced at high output power in the case wherethe capacitor 240 is not provided.

FIG. 12 is a graph illustrating simulation results when the capacitancevalue C_(CUT) of the capacitor 210 is 0.4 pF and the capacitance valueC_(ADD) of the capacitor 240 is 1 pF. The abscissa and the ordinate inFIG. 12 are the same as in FIG. 11.

As indicated at point D2 in FIG. 12, when the transistor 200 is turnedoff, current (negative current I_(ADD)) flows from the capacitor 240into the base of the transistor 200. As indicated at point A2, thiscurrent makes the amount of decrease in base voltage V_(B) at highoutput power smaller than that in FIG. 11. Along with this, as indicatedat point B2, the amount of increase in bias current I_(BIAS) is alsomade smaller than that in FIG. 11. This suppresses the timing of turningon the transistor 200 from becoming earlier than that in FIG. 11 asindicated at point C2. Thus, when the capacitor 240 is provided, it isfound that the power added efficiency is improved at high output power.

FIG. 13 is a graph illustrating simulation results that indicate anexample of the relationship between the capacitance value C_(ADD) of thecapacitor 240 and power added efficiency in the power amplifier 160B. InFIG. 13, the abscissa indicates output level (dBm) and the ordinateindicates power added efficiency (%). As illustrated in FIG. 13, whenthe capacitor 240 is not provided (in the case of C_(ADD)=0.01 pF), thepower added efficiency starts to decrease largely from an output levelof 30 dBm. On the other hand, the addition of the capacitor 240 cansuppress the decrease in power added efficiency at high output power.Particularly, in the example illustrated in FIG. 13, when thecapacitance value C_(ADD) is set to 0.8 pF to 1.2 pF (roughly equivalentor substantially similar to the capacitance value of the transistor 200in the off state), power added efficiency at high output power issignificantly improved.

Next, simulation results when the capacitance value C_(CUT) of thecapacitor 210 is increased to support a broadband RF signal will bedescribed. FIG. 14 is a graph illustrating simulation results when thecapacitance value C_(CUT) of the capacitor 210 is 1.4 pF and thecapacitance value C_(ADD) of the capacitor 240 is 0.01 pF. The abscissaand the ordinate in FIG. 14 are the same as in FIG. 11.

As indicated at point A3 in FIG. 14, when the transistor 200 is turnedoff at high output power, the base voltage V_(B) significantly drops.Along with this, the bias current I_(BIAS) increases as indicated atpoint B3. The higher the bias current I_(BIAS), the earlier the timingat which the transistor 200 is turned on as indicated at point C3. Fromthis, it is found that power added efficiency is reduced at high outputpower in the case where the capacitor 240 is not provided.

FIG. 15 is a graph illustrating simulation results when the capacitancevalue C_(CUT) of the capacitor 210 is 1.4 pF and the capacitance valueC_(ADD) of the capacitor 240 is 1 pF. The abscissa and the ordinate inFIG. 15 are the same as in FIG. 11.

As indicated at point D4 in FIG. 15, when the transistor 200 is turnedoff, current (negative current I_(ADD)) flows from the capacitor 240into the base of the transistor 200. As indicated at point A4, thiscurrent makes the amount of decrease in base voltage V_(B) at highoutput power smaller than that in FIG. 14. Along with this, as indicatedat point B4, the amount of increase in bias current I_(BIAS) is alsomade smaller than that in FIG. 14. This suppresses the timing of turningon the transistor 200 from becoming earlier than that in FIG. 14 asindicated at point C4. When the capacitor 240 is provided, it is foundthat the power added efficiency is improved at high output power. Thus,when the capacitor 240 is provided regardless of the capacitance valueof the capacitor 210, it is found that power added efficiency isimproved.

FIG. 16 is a diagram illustrating an example of a structure in which thecapacitor 240 is made up using diodes. As illustrated in FIG. 16, thecapacitor 240 can be made up of multiple diodes 1000 connected inparallel. Specifically, as illustrated in FIG. 16, the capacitor 240 canbe made up by connecting the cathodes of the diodes 1000 connected inparallel to the base of the transistor 200 and connecting the anodes ofthe diodes 1000 to the emitter of the transistor 200. Compared with thestructure of the capacitor 240 set as MIM capacitance, the capacitor 240made up of multiple diodes 1000 connected in parallel can decrease achip size when the power amplifier 160 is integrated.

The embodiments of the present disclosure have been described above. Asdescribed above, according to the embodiments, the capacitor 240 withone end connected to the base of the transistor 200 and the other endconnected to the emitter of the transistor 200 is provided to enable animprovement in the power added efficiency of the power amplifier 160 athigh output power.

Further, according to the embodiments, the capacitance value C_(ADD) ofthe capacitor 240 is set to be roughly equivalent or substantiallysimilar to the capacitance value of the transistor 200 in the off state,and this can significantly improve power added efficiency at high outputpower.

Note that each of the embodiments described above is to make it easy tounderstand the present disclosure, and should not be interpreted tolimit the present disclosure. The present disclosure can be modified andimproved without departing from the spirit of the disclosure, andequivalents thereof are included in the present disclosure. In otherwords, each of the embodiments subjected appropriately to design changeby those skilled in the art is included in the scope of the presentdisclosure as long as it has substantially the same features of thepresent disclosure. For example, each component included in eachembodiment, and the arrangement, material, condition, shape, size, andthe like thereof are not limited to illustrated ones, and can beappropriately changed. Further, respective components included inrespective embodiments can be combined if technically possible, and thecombined components are included in the scope of the present disclosureas long as they have substantially the same features of the presentdisclosure.

For example, the bias circuit 220 is formed as an emitter-followercircuit by the transistor 250 in the embodiments, but the structure ofthe bias circuit 220 is not limited thereto. Specifically, any structurecan be employed for the bias circuit 220 as long as the bias currentI_(BIAS) increases with a decrease in the base voltage V_(B) of thetransistor 200.

Further, the example of providing the capacitor 240 in the poweramplifier 160 as the power stage of the power amplification module 120is described in the embodiments, but a configuration equivalent to thatof the power amplifier 160 can also be employed in the power amplifier150 as the drive stage. The same holds for a configuration havingthree-stage power amplifiers or more.

DESCRIPTION OF REFERENCE NUMERALS

100 transmitting unit

110 modulation unit

120 power amplification module

130 front end unit

140 antenna

150, 160 power amplifier

170, 180, 190 matching circuit

200, 250 transistor

210, 240, 280 capacitor

220 bias circuit

230 Inductor

260, 270 resistor

290, 291, 1000 diode

300 unit cell

What is claimed is:
 1. A power amplifier comprising: at least two transistors connected in parallel for amplifying a radio frequency signal; and a capacitor arranged at each of the transistors, wherein: a first end of the capacitor is directly connected to a base of each of the transistors and a second end of the capacitor is connected to an emitter of each of the transistors, and the capacitor is a metal insulator metal capacitor.
 2. The power amplifier according to claim 1, wherein: each of the transistors and the capacitor are formed on a single chip.
 3. The power amplifier according to claim 1, wherein: the capacitor is made up of at least one diode, and a cathode of the diode is connected to the base of each of the transistors and an anode of the diode is connected to the emitter of each of the transistors.
 4. The power amplifier according to claim 2, wherein: the capacitor is made up of at least one diode, and a cathode of the diode is connected to the base of each of the transistors and an anode of the diode is connected to the emitter of each of the transistors.
 5. The power amplifier according to claim 1, wherein a capacitance value of the capacitor is substantially similar to a capacitance value of each of the transistors in an off state.
 6. The power amplifier according to claim 2, wherein a capacitance value of the capacitor is substantially similar to a capacitance value of each of the transistors in an off state.
 7. The power amplifier according to claim 3, wherein a capacitance value of the capacitor is substantially similar to a capacitance value of each of the transistors in an off state.
 8. The power amplifier according to claim 4, wherein a capacitance value of the capacitor is substantially similar to a capacitance value of each of the transistors in an off state.
 9. The power amplifier according to claim 1, wherein a capacitive value of the capacitor is between 0.8 pF and 1.2 pF.
 10. The power amplifier according to claim 2, wherein a capacitive value of the capacitor is between 0.8 pF and 1.2 pF.
 11. The power amplifier according to claim 3, wherein a capacitive value of the capacitor is between 0.8 pF and 1.2 pF.
 12. The power amplifier according to claim 4, wherein a capacitive value of the capacitor is between 0.8 pF and 1.2 pF. 